已知A=x的平方-2xy+y的平方,B=2x平方-6xy+3y平方 求3A-[2A-B]-4(A-B)的值|x|=5,x

已知A=x²-2xy+y²,B=2x²-6xy+3y²
3A-(2A-B)-4(A-B)
=3A-2A+B-4A+4B
=-3A+5B
=-3(x²-2xy+y²)+5(2x²-6xy+3y²)
=-3x²+6xy-3y²+10x²-30xy+15y²
=7x²-24xy+12y²
|x|=5,x+y=-2
①
x=-5,y=3
原式=7*(-5)²-24*(-5)*3+12*3²=643
②
x=5,y=-7原式=7*5²-24*5*(-7)+12*(-7)²=1603

3A-[2A-B]-4(A-B)=5B-3A=7x平方-24xy+12y平方
x=5，y=-7；x=-5，y=3
7x平方-24xy+12y平方=1630或643

是不是：x-y=-2
3A-[2A-B]-4(A-B)
=3A-2A+B-4A+4B
=5B-3A
=5(2x²-6xy+3y²)-3(x²-2xy+y²)
=7x²-24xy+12y²
=12x²-24xy+12y²-5x²
=12(x-y)²-5x²
=12*(-2)²-5*5²
=-77

A=x^2-2xy+y^2=(x-y)^2
B=2x^2-6xy+3y^2=-x^2+3(x^2-2xy+y^2)=3(x-y)^2-x^2
3A-[2A-B]-4(A-B)=3A-2A+B-4A+4B=5B-3A=15(x-y)^2-5x^-3(x-y)^2=12(x-y)^2-5x^2
题目的 x+y=-2, 如果是x-y=-2, 代入可得：
=12*4-5*25=-77